Polyester (particularly, polyethylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate, etc.) resin is excellent in mechanical characteristic, thermal characteristic, chemical resistance, electric characteristic and formability, and is used for various uses. A polyester film obtained by making the polyester resin into a film shape, in particular, a biaxially oriented polyester film, because of its mechanical characteristic, electric characteristic, etc., is used as electrical insulation materials for a copper clad laminate, a solar battery backsheet, an adhesive tape, a printed circuit board, a membrane switch, a planar heat generator element, a flat cable, etc. as well as a magnetic recording material, a capacitor-purpose material, a wrapping material, a motor vehicle-purpose material, a building material, and various industrial materials for a photographic use, a graphical use, a thermosensitive transcription use, etc.
Among these uses, the uses as an electrical insulation material (e.g., a solar battery backsheet or the like), a motor vehicle-purpose material, a building material, etc., are often made in environments that are severe in temperature and moisture over long periods. However, since a general-purpose polyester resin reduces in molecular weight due to hydrolysis and also reduces in mechanical characteristic due to progress of embrittlement, betterment in that aspect, that is, improvement in moist heat resistance, is demanded.
To that end, various studies for restraining the hydrolysis of the polyester resin have been carried out. For example, a polyester resin (Japanese Patent Application Publication No. Sho 60-31526) that contains an alkali metal, an alkaline earth metal, and phosphorus and that contains internally deposited particles from catalyst residues, and a technology of improving the moist heat resistance of the polyester resin itself by adding an epoxy compound (Japanese Patent Application Publication Nos. Hei 9-227767 and 2007-302878) or a polycarbodiimide (Published Japanese Translation of PCT Application No. Hei 11-506487) have been studied. Besides, as for a biaxially oriented polyester film, studies have been done, such as a study for improving the moist heat resistance by providing the film with high IV (high intrinsic viscosity) and controlling the degree of plane orientation (Japanese Patent Application Publication No. 2007-70430).
On another hand, for application to the aforementioned uses, it is desired that multifunctionality of the material be realized by providing the material with characteristics besides the moist heat resistance (e.g., sliding characteristic, ultraviolet ray resistance, reflection characteristic, etc.). To that end, a polyester of two or more components, a mixture of polyester with another component for realizing more functions have been studied (e.g., Japanese Patent Application Publication Nos. 2003-155403, Hei 2-163155 and Hei 2-191638).
However, if polyester is mixed with another component (e.g., an ultraviolet absorbing agent, an inorganic particle, etc.) to realize multifunctionality of a polyester film and, in particular, a polyester film whose main component is an ethylene terephthalate unit, degradation of the material due to hydrolysis or the like progresses during kneading, giving rise to a problem of the resultant film having a reduced moist heat resistance while developing the function of the added component.
Accordingly, it could be helpful to provide a polyester film that is excellent in moist heat resistance and also capable of achieving another characteristic, particularly, ultraviolet ray resistance, light reflecting characteristic, etc.